1. Field of the Invention
The present invention discloses an improved process for oxidizing alkyl aromatic hydrocarbons and/or their partially oxidized intermediates to produce aromatic carboxylic acids. The process involves the liquid phase oxidation in the presence of a catalyst of cobalt-manganese-bromine in an aliphatic carboxylic acid having 1-6 carbon atoms such as acetic acid as a solvent with a gas containing oxygen and carbon dioxide. Furthermore, an additional transition metal or lanthanide series metal is introduced to the cobalt-manganese-bromine catalyst system, when deemed necessary.
The rate of the oxidation reaction of an alkyl aromatic substrate was remarkably increased in the present process over the conventional oxidation process. The yield and quality of the carboxylic acid products were also significantly improved in the process. Thus, for example, terephthalic acid and isophthalic acid of improved yield and purity are produced by carrying out the oxidation of para-xylene and meta-xylene, respectively, in the co-presence of carbon dioxide and oxygen under relatively mild reaction conditions.
2. Description of the Related Art
As discussed below, methods of manufacturing aromatic carboxylic acids are well known and widely used commercially. For example, the method of manufacturing of aromatic carboxylic acids such as terephthalic acid (TPA), isophthalic acid (IPA), phthalic acid, phthalic anhydride, naphthalene dicarboxylic acid, trimellitic acid, trimellitic anhydride, trimesic acid, pyromellitic dianhydride, 4,4'-biphenyldicarboxylic acid and benzoic acid by oxidizing alkylaromatic compounds or the oxidized intermediates thereof, in the presence of cobalt-manganese-bromine, from such alkylaromatic compounds as para-xylene, para-tolualdehyde, para-toluic acid, 4-carboxybenzaldehyde (4-CBA), meta-xylene, meta-tolualdehyde, meta-toluic acid, 3-carboxybenzaldehyde, ortho-xylene, dimethylnaphtalene, pseudocumene (1,2,4-trimethylbenzene), mesitylene (1,3,5-trimethylbenzene), durene (1,2,4,5-tetramethylbenzene), 4,4'-dimethylbiphenyl and toluene is well known (for example, U.S. Pat. Nos. 2,833,816 and 5,183,933). Such aromatic carboxylic acids are used as raw materials for manufacturing polyester after appropriate purification such as hydrogenation, etc. (U.S. Pat. No. 3,584,039). Also, polyester is widely used as a synthetic fiber, film, etc.
There were continuous endeavors to develop a catalyst system with high efficiency and enhanced reactivity to manufacture aromatic carboxylic acids. The newly developed technologies, however, were not practical due to the increase in side reactions, price of catalyst, difficulty of operation, precipitation of catalyst, etc.
Improvements in the efficiency of the reaction for manufacturing of aromatic carboxylic acids are very significant because they may improve productivity, quality and cost competitiveness due to the reduction in the reaction time and side reactions. In other words, it is highly desirable to develop a technology to increase the efficiency of the oxidation reaction of alkyl aromatic compounds and the oxidized intermediates thereof by means of an improvement in the reaction processes.
There were many attempts to increase the efficiency by adding a third metal catalyst to the cobalt-manganese-bromine catalyst system which is the basic catalyst system, to enhance the catalyst efficiency during the manufacturing of aromatic carboxylic acids. The added metals were mainly transition metals, and by adding, for example, hafnium, zirconium, molybdenum, etc., the reactivity therein was increased (U.S. Pat. No. 5,112,992).
On the other hand, an oxygen containing gas such as air was mainly used as an oxidant during the manufacturing of aromatic carboxylic acids. Carbon dioxide was not used as an oxidant due to its chemical stability. Yet, in the research for improving the process efficiency, there was a case in which chemically stable carbon dioxide, recycled from the reaction vent gas, was injected to the reactor to increase the stability in the process by mitigating the problematic possibilities of explosion due to oxygen when using pure oxygen or gas containing pure oxygen or oxygen enriched gas as an oxidant (U.S. Pat. No. 5,693,856). Nevertheless, the case is not known in which carbon dioxide was added to improve the reaction efficiency and the effects of the concentration of added carbon dioxide on oxidation.
In summary, the basic oxidation technology for manufacturing carboxylic acids, especially for TPA manufacture, has been extensively developed. The basic process technology is now approaching a point of diminishing returns, and further major breakthroughs i.e., new catalyst systems, raw materials, and basic unit operations, are not anticipated. The leading producers are expected to have greater optimization and energy integration across the entire production complex with more advanced control schemes. However, surpassing the current general expectation, the present invention have made a remarkable breakthrough to achieve the improved catalyst activity and selectivity toward aromatic carboxylic acids, especially for terephthalic acid and isophthalic acid, in the aforementioned catalyst composition under milder oxidation conditions.